This application claims priority to PCT Application No. GB98/03007 filed Oct. 7, 1998 which claims priority to GB Application No. 9721400.1 filed Oct. 9, 1997.
This application relates to the construction of underground structures in tunnel excavations without causing surface disruption. This type of excavation technique has been developed in the last 30 years and there is a growing need to install structures such as, for example, traffic underpasses, below an existing rail track or highway without stopping the use and operation of the same. Another example is the creation of a metro station below a busy street or property.
The problem with traditional tunnelling techniques is that for safety reasons there is required to be a depth of soil of approximately 2 to 3 times the diameter of the tunnel which is to be excavated, above the said tunnel. This renders the traditional techniques impractical and so a number of conventional methods have been developed and are now used which reduce the requirement for such a great depth of soil to be provided above the tunnel. These methods are based on the principle of jacking pre-cast structure units into the excavated area, as the same is excavated to form a structure as the tunnel is formed. The formation of the structure allows the support of the tunnel as it is formed without the need to cause disruption to services or property on the surface.
A known approach is to prepare the structure to be installed at the side of the excavation and then jack it horizontally into position in the excavation. This has the disadvantage of requiring large constructions to be formed at the side and an extended area to be prepared for carrying out the work, usually of at least the same dimensions as the installation. It is also a process that is time consuming as a great deal of preparatory work has to be done in forming the working areas and casting the structure units.
A second known approach is a modular approach where a series of pre-cast units are jacked, one on top of another, to form piers and abutments. This is a system which has found extensive use but has the disadvantage of not providing a complete solution to the problem as, although the majority of the excavation work can be completed without disruption it is necessary at some stage to complete the work by taking possession of the excavation so as to allow installation of the spanning beams.
A third known approach is to create a structure of arch shaped cross section which is formed by a series of relatively small section tubes which run along the length of the structure. This provides a canopy which allows excavation to take place safely underneath. The disadvantages with this is that it is difficult and expensive to place all the tubes in position and, normally it is necessary to provide props for the arch across the base of the same and put in temporary support beams to support the tube arch and these procedures are required to be undertaken as work progresses.
Documents DE3609791 and U.S. Pat. No. 3,916,630 both disclose methods of formation of support structures with DE3609791 disclaiming the formation of a pipe structure and U.S. Pat. No. 3,916,630 the formation of a structure cast in situ; however neither discloses the formation of an arch structure from units pushed or jacked into the excavation.
The aim of the present invention is to provide an improved process of supporting material excavations by utilising a modular pre-cast unit based on the principle of using units formed of an arch shape such that a series of said units allow an arch structure to be formed, said arch being an efficient form of carrying live and dead loads and therefore well suited to creating an underground structure. The approach is to pre-cast arch panels, erect them in the excavated area and jack the assembled elements forward to form the structure.
In a first aspect of the invention there is provided a support structure which can be used to support excavated areas during and/or following excavation, said support structure including a series of upstanding arch shaped sections, positioned along the length of the excavated area, one after the other, and characterised in that said arch sections are pushed or jacked in an upstanding position into the excavated area.
In a preferred embodiment the support structure is formed with arch section ends being located in and along a series of supporting units. In one preferred embodiment the units have recessed sections, which, when the units are laid end to end, form a track along which the arch sections can slide when jacked. Typically, two linear tracks are formed, said tracks spaced apart by a distance determined by the space between the ends of said arch sections.
Typically, the arch sections and/or supporting units are pre-cast. Yet further, each of the arch sections are formed from a series of panels, constructed on site and prior to insertion into the tunnel.
In a further aspect of the invention, there is provided a method for forming a support structure for an excavated area during and/or after excavation of the same, said method comprising, as the tunnel is excavated, pushing or jacking a series of sections in an upstanding position one after another into said excavated area, characterised in that the sections are arch sections in order to form an arch shaped support structure.
Typically the excavated area is a tunnel and the method comprises the steps of jacking a series of arch sections at intervals to increase the length of the support structure into the tunnel as the tunnel is excavated. The activity of the tunnel excavation takes place to the front of the first of the arch sections introduced.
In one embodiment, supporting units are first positioned in the excavation to act as bases and guides along which the arch structures are introduced.
In one embodiment, the supporting units extend upwardly to form the side walls of the arch shaped structure and it is the curved arch sections which are introduced to form the arch shaped structure. Alternatively the arch sections include both the roof and side walls when jacked into the excavation.
The method of the invention has a number of technical and economic advantages. Arch sections can be formed from a number of panels by factory fabrication, delivered to site and connected together to form the arch.
In one embodiment a temporary shield can be fitted at the leading face, i.e. in front of the first arch section, which allows excavation work to be undertaken safely. This shield is recovered at the end of the excavation and can be re-used for excavations thereafter.
Similarly, a shield can be provided at the front of each supporting unit to allow excavation to proceed safely.
The use of arch panels reduces the temporary working areas required at the excavation site and requires less heavy handling equipment, than with conventional techniques.
Typically, the ends of the panel sections are located in tracks formed by a series of supporting units which are jacked into the tunnel and the method further includes the step of jacking said supporting units into the tunnel to provide tracks of a sufficient length to receive the arch sections to form the support structure and therefore may be advanced to a further position into the excavation than the arch sections. Typically, the units are required to be manipulated after jacking to expose recessed portions to allow the formation of the tracks.
To further improve the structure, hydrophilic gaskets or groutable injection hoses can be introduced, between panels as they are installed in the working pit which serve to waterproof the joints and it should be appreciated that there are many possible variations of details in the design of the foundations and the arch configuration and span.
In one embodiment double, side by side arched structures can be created, for example, for a tunnel for the two carriageways of a divided highway. In one embodiment three or four sets of in line supporting units are provided, said supporting units comprising two lines of outer supporting units and a centre line of double units and/or single units having two guide tracks formed therein, thus allowing the introduction of two sets of side by side sections along said supporting units.
As an alternative embodiment to the use of supporting units in block form there is provided the method of forming tunnels, typically of circular cross section, along the line of the support structure to be formed and said tunnels spaced apart by the spacing required for the arch sections. The tunnels are driven by jacking or by segment construction. In each tunnel there is formed a track for the reception of the ends of the arch sections which again pass along the length of the tracks as with the supporting units and therefore act in a similar manner to support the arch sections.